In recent years, printing employing a CTP system has been conducted in printing industries, accompanied with the digitization of printing data. A printing plate material for CTP, which is inexpensive, can be easily handled, and has a printing ability comparable with that of a PS plate, is required.
A versatile processless printing plate has been sought, which has a direct imaging (DI) property not requiring any development employing a specific developer, can be applied to a printing press with a direct imaging (DI) function, and can be handled in the same manner as in PS plates.
A thermal processless printing plate material is imagewise exposed employing an infrared laser with an emission wavelength of from near-infrared to infrared regions to form an image. The thermal processless printing plate material employing this method is divided into two types; an ablation type printing plate material and an on-press development type printing plate material with a heat melting image formation layer.
Examples of the ablation type printing plate material include those disclosed in for example, Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773.
These references disclose a printing plate material comprising a support, and provided thereon, a hydrophilic layer and a lipophilic layer, either of which is an outermost layer. When a printing plate material is imagewise exposed in which the hydrophilic layer is an outermost layer, the hydrophilic layer is removed by ablation to reveal the lipophilic layer, whereby an image is formed. This printing plate material has problem that the exposure device used is contaminated by the ablated matter, and a special suction device is required for removing the scattered material. Therefore, this printing plate material is low in versatility to the exposure device.
A printing plate material has been developed which is capable of forming an image without ablation, and does not require development treatment employing a special developer or wiping-off treatment. There is, for example, a printing plate material for CTP as disclosed in Japanese Publication Nos. 2938397 and 2938397, which comprises an image formation layer containing thermoplastic particles and a water-soluble binder and which is capable of be developed with a dampening solution or printing ink on a printing press.
However, the thermoplastic particles may be slightly heat-fused even at a relatively low temperature such as 50 to 60° C., and a printing plate material comprising such thermoplastic particles may lower on-press developability (developability on a printing press) after storage at 50 to 60° C., resulting in insufficient storage stability.
As a thermosensitive image formation material, a blocked isocyanate compound is known. The blocked isocyanate compound has a blocked isocyanate group in which the isocyanate group is chemically protected by a specific blocking material, and is non-reactive at not more than a specific temperature (generally 100° C.).
The blocked isocyanate compound releases the blocking material at not less than the specific temperature to reproduce an isocyanate group, and is reactive. The releasing temperature of the blocking material has a clear threshold and the blocked isocyanate compound provides good storage stability at a temperature not more than the threshold.
There is a printing plate material comprising an image formation layer containing a blocked isocyanate compound wherein the image formation layer or a layer adjacent thereto contains a compound having a functional group reacting with isocyanate group. The printing plate material is imagewise heated in which the isocyanate compound is cross-linked at the heated portions, producing a difference in ink receptivity or dampening solution receptivity on the printing plate material surface. Thus, a printing plate is prepared.
For example, a planographic printing plate material is proposed (in for example, Japanese Patent O.P.I. Publication No. 62-164049) which comprises a hydrophilic support and provided thereon, a recording layer containing a blocked isocyanate compound and a polymer having an active hydrogen capable of reacting with an isocyanate compound wherein at least one of the support and the recording layer contains a light-to-heat conversion material.
As another example, there is a planographic printing plate material disclosed in Japanese Patent O.P.I. Publication No. 2001-310566), which comprises three-dimensionally cross-linked hydrophilic polymer and a specific blocked isocyanate compound dispersed in the hydrophilic polymer, in which a functional group of the hydrophilic polymer reacts with the isocyanate compound to form a hydrophobic image at the heated portions. This plat material has advantage that development including on-press development is unnecessary, but has problem in that since the hydrophilic layer contains a hydrophobic material such as a blocked isocyanate compound, hydrophilicity of the hydrophilic layer is lowered in non-heated portions, and the hydrophilic is likely to be rendered hydrophobic due to pressure, resulting in stain occurrence at non-image portions and even at slightly scratched non-image portions.
As an example applied to on-press development in a CTP system, there is a printing plate material comprising a hydrophilic support, and provided thereon, a thermosensitive layer containing hydrophobic polymer particles, a blocked isocyanate compound, a hydrophilic resin having a group capable of reacting with an isocyanate compound, and a light-to-heat conversion material (see Japanese Patent O.P.I. Publication No. 2002-225451), or a printing plate material comprising a hydrophilic support, and provided thereon, a thermosensitive layer containing polymer particles having a blocked isocyanate group, a hydrophilic resin, and a light-to-heat conversion material (see Japanese Patent O.P.I. Publication No. 2002-283758).
However, these printing plate materials are not satisfactory in minimizing printing performance after storage, while maintaining excellent initial printability and high printing durability.